1. Field of the Invention
This invention relates to a switching power circuit and power control method.
2. Description of the Related Art
A switching power circuit such as a DC-DC converter generates a desired Voltage from a power source Voltage by controlling a switching element. At this time, the switching element is controlled very often by DPWM (Digital Pulse Width Modulation). In the DPWM control, a counter is used (for example, JP-A 2004-304872 (KOKAI)). This is because the construction of circuit is relatively simple and the DPWM control can be realized relatively at low cost.
However, a DC-DC converter controlled by DPWM is efficient at heavy load, but is inefficient at light load. In contrast, a DC-DC converter controlled by DPFM (Digital Pulse Frequency Modulation) is efficient at light load, but is inefficient at heavy load. Owing to this, it is desirable that the switching of the DPWM control and the DPFM control would be possible. Now, when a counter system is used in the DPWM control, switching frequency is determined by time of which all bits of counter come from 0 to 1. Thus, when the counter is used, it is difficult to change the switching frequency, that is, to coexist the DPWM control and the DPFM control together.